Aqueous composition

ABSTRACT

The present invention relates to an aqueous composition comprising anionic organic polymeric particles and colloidal anionic silica-based particles. The anionic organic polymeric particles are obtainable by polymerising one or more ethylenically unsaturated monomers together with one or more polyfunctional branching agents and/or polyfunctional crosslinking agents. The anionic organic polymeric particles and the colloidal anionic silica-based particles in the aqueous composition are present in an amount of at least 0.01% by weight based on the total weight of the aqueous composition. The present invention further relates to a method for the preparation of the aqueous composition, uses of the aqueous composition and a process for the production or paper, in which the aqueous composition is added to a cellulosic suspension.

[0001] The present invention relates to an aqueous compositioncomprising anionic organic polymeric particles and colloidal anionicsilica-based particles, a method for the preparation of the composition,uses of the composition and a process for the production or paper.

BACKGROUND OF THE INVENTION

[0002] In the papermaking art, an aqueous suspension containingcellulosic fibres, and optional fillers and additives, referred to asstock, is fed into a headbox which ejects the stock onto a forming wire.Water is drained from the stock through the forming wire so that a wetweb of paper is formed on the wire, the paper web is further dewateredin the press section, and dried in the drying section of the papermachine. Drainage and retention aids are conventionally introduced intothe stock in order to facilitate drainage and to increase adsorption offine particles onto the cellulosic fibres so that they are retained withthe fibres on the wire.

[0003] U.S. Pat. Nos. 4,750,974 and 4,643,801 discloses a coarcervatebinder for use in a papermaking process. First cationic potato starch isadded into the process, then an anionic mixture comprising anionicpolymer and silica is added. The mixture contains 20:1 to 1:10 ofanionic polymer to silica.

[0004] U.S. Pat. No. 6,083,997 discloses anionic nano-composites, whichare prepared by adding a polyelectrolyte to silicate solution and thencombining them with silicic acid. The nano-composites exhibit improvedretention and drainage performance in papermaking processes.

[0005] U.S. Pat. No. 5,167,766 discloses a papermaking process in whichan ionic organic polymer microbead is used together with a syntheticorganic polymer or polysaccharide as retention and drainage aids

[0006] It would be advantageous to be able to provide drainage andretention aids with improved performance. It would also be advantageousto be able to provide retention and drainage aids with good storagestability. It would further be advantageous to be able to provide apapermaking process with improved drainage and/or retention performance

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention there is provided anaqueous composition comprising anionic organic polymeric particles andcolloidal anionic silica-based particles. The anionic organic polymericparticles being obtainable by polymerising ethylenically unsaturatedmonomers together with a polyfunctional branching agent and/or apolyfunctional crosslinking agent and the amount of the anionic organicpolymeric particles together with the colloidal anionic silica-basedparticles is at least 0.01% by weight based on the total weight of theaqueous composition.

[0008] Also, according to the present invention, there is provided amethod for preparation of an aqueous composition. The method comprisesmixing colloidal anionic silica-based particles with anionic organicpolymeric particles obtainable by polymerising ethylenically unsaturatedmonomers together with a polyfunctional branching agent and/or apolyfunctional crosslinking agent, obtaining an amount of the colloidalanionic silica-based particles together with the anionic organicpolymeric particles in the composition of at least 0.01% by weight basedon the total weight of the aqueous composition.

[0009] There is further provided a process for production of paper froman aqueous suspension containing cellulosic fibres, and optionalfillers. The process comprises adding to the fibre suspension a cationicpolymer and an aqueous composition comprising anionic organic polymericparticles and colloidal anionic silica-based particles, the anionicorganic polymeric particles being obtainable by polymerisingethylenically unsaturated monomers together with a polyfunctionalbranching agent and/or a polyfunctional crosslinking agent, the amountof anionic organic polymeric particles and colloidal anionicsilica-based particles being at least 0.01% by weight based on the totalweight of the aqueous composition.

[0010] There is also according to the present invention provided uses ofan aqueous composition as flocculating agent, retention and drainageaids. There is further provided a paper obtainable by the process forthe production of paper from an aqueous suspension containing cellulosicfibres.

DETAILED DESCRIPTION OF THE INVENTION

[0011] According to the present invention it has unexpectedly been foundthat improved drainage and/or retention effect in papermaking can beobtained by using colloidal anionic silica-based particles incombination with anionic organic polymeric particles.

[0012] The term “drainage and retention aid”, as used herein, refers toone or more components, which when added to an aqueous cellulosicsuspension, give better drainage and/or retention than is obtained whennot adding any of said one or more components.

[0013] The aqueous composition according to the invention comprisesanionic organic polymer particles and colloidal anionic silica-basedparticles. The anionic organic polymeric particles can be obtained bypolymerising one or more anionic monomers and optionally one or morenonionic monomers capable of forming homopolymers or copolymers. Themonomers forming the anionic organic polymeric particles can be formedfrom one or more anionic monomers such as anionic ethylenicallyunsaturated monomers selected from (meth)acrylic acids,2-acrylamido-2-methylpropane sulfonate, sulfoethyl (meth)acrylate,vinylsulfonic acid, sulfoalkyl (meth)acrylic acids, sulfonated styrenes,unsaturated dicarboxylic acids, maleic or other dibasic acids,sulfoalkyl (meth)acrylamides, salts of said acids such as alkali orammonium salts and mixtures thereof.

[0014] The anionic organic polymeric particles can be formed bycopolymerising one or more of the above anionic monomers with one ormore nonionic monomers such as (meth)acrylamides, N-alkylacrylamidessuch as N-methylacrylamide, N,N-dialkyl-acrylamides such asN,N-dimethylacrylamide, vinylacetate, alkyl (meth)acrylates such asmethylacrylate, methylmethacrylate, acrylonitrile,N-vinylmethylacetamide, N-vinylmethylformamide, vinylacetate or N-vinylpyrrolidone mixtures of any of the foregoing and the like. Theethylenically unsaturated nonionic monomers may be copolymerised asmentioned above to produce anionic copolymers, preferably acrylamide iscopolymerised with an anionic monomer.

[0015] The particles may be formed from a branched and/or crosslinkedanionic polymer of anionic monomers alone or copolymensed with nonionicmonomers. The polymerisation of the particles can occur by using apolyfunctional branching agent and/or a polyfunctional crosslinkingagent, optionally in the presence of a chain-transfer agent.Polymerisation of such monomers to form particles are known from U.S.Pat. Nos. 5,961,840, 5,919,882, 5,171,808 and 5,167,766.

[0016] Useful polyfunctional crosslinking or branching agents comprisecompounds having either at least two ethylenically unsaturated bonds orat least one ethylenically unsaturated bond and at least one reactivegroup or at least two reactive groups

[0017] Examples of suitable agents having at least two ethylenicallyunsaturated bonds include N,N-methylene-bis(meth)acrylamide,polyethyleneglycol di(meth)acrylate, N-vinylacrylamide, divinylbenzene,triallylammonium salts, N-methylallylacrylamide, and the like.

[0018] Examples of suitable agents having at least one ethylenicallyunsaturated bond and at least one reactive group includeglycidyl(meth)acrylate, acrolein, methylolacrylamide and the like.

[0019] Examples of suitable branching or crosslinking agents having atleast two reactive groups include dialdehydes such as glyoxal,epichlorohydrin, diepoxy compounds and the like

[0020] Suitably a molecular weight modifying or chain-transfer agent isused in the polymerisation to control the structure of the polymerSuitable chain-transfer agents, which can be used to produce theparticles include alcohols, mercaptans, thioacids, phosphites andsulphites, such as isopropyl alcohol and sodium hypophosphite, althoughmany different chain-transfer agents may be employed The polymerisationprocess suitably comprises the steps of:

[0021] i) preparing a monomer emulsion by adding the monomers containedin aqueous phase to a hydrocarbon liquid containing a suitablesurfactant or surfactant mixture;

[0022] ii) forming an inverse monomer emulsion of small aqueous dropletsdispersed in an oil phase; and

[0023] iii) polymerising the monomers in the emulsion droplets by freeradical polymerisation.

[0024] The aqueous phase contains anionic monomers and/or nonionicmonomers together with branching and/or crosslinking agents. Preferablyanionic organic polymeric particles have an unswollen average particlediameter size of less than about 750 nanometers, preferably less than500 nm, more preferably from about 25 to about 300 nm.

[0025] The polyfunctional branching or crosslinking agents shouldpreferably be used in sufficient quantities to induce sufficientbranching of the polymeric product and/or sufficient crosslinks in thepolymeric product. A suitable content of the polyfunctional branchingand/or crosslinking agents can be at least 4 molar parts per millionbased on the monomeric units present in the polymer, preferred is acontent of agents from about 4 to 6000 molar parts per million, morepreferred about 20 to 4000, and most preferred about 50 to 2000 molarparts per million.

[0026] The anionic organic polymeric particles can contain from about 0to about 99 parts by weight of nonionic monomers and from about 100 toabout 1 parts by weight of anionic monomers based on the total weight ofanionic and nonionic monomers, preferably from about 10 to about 90parts by weight of nonionic monomers and from about 90 to about 10 partsby weight of anionic monomers, more preferably from about 20 to about 80parts by weight of nonionic monomers and from about 80 to about 20 partsby weight of anionic monomers. The anionic organic polymeric particleshaving a charge of at least 2 meq/g, suitably at least within the rageof 2 to 18 meq/g, preferably within the range 3 to 15 meq/g, and morepreferably within the range of 5 to 12 meq/g.

[0027] The aqueous composition according to the invention also comprisescolloidal anionic silica-based particles, i.e. colloidal silica such asdifferent types of polysilicic acid, silica particles, which aremodified and contain other elements or compounds e.g. amine, aluminiumand/or boron, which can be present in the aqueous phase and/or in thesilica particles, and mixtures thereof. In the art, polysilicic acid isalso referred to as polymeric silicic acid, polysilicic acid microgel,polysilicate and polysilicate microgel, which are all encompassed by theterm polysilicic acid used herein. Aluminium-containing compounds ofthis type are commonly referred to as colloidal aluminium-modifiedsilica including polyaluminosilicate and polyaluminosilicate microgel,which are encompassed by the term colloidal aluminium-modified silica,which is used herein. Preferred silica-based particles can besurface-modified with aluminium to a degree of from 2 to 25%substitutions of silicon atoms Suitable colloidal silica-based particlesare disclosed in U.S. Pat. Nos. 5,643,414, 5,603,805 and 5,447,604, andin International Patent Applications WO 00/66491, WO 00/66492 and WO01/46072. Mixtures of suitable silica-based particles can also be used.

[0028] It is preferred that the anionic silica-based particles are inthe colloidal range of particle size. The average particle size of theanionic silica-based particles is suitably below about 50 nm, preferablybelow about 20 nm and more preferably in the range of from about 1 toabout 10 nm. As conventional in silica chemistry, the particle sizerefers to the average size of the primary particles, which may beaggregated or non-aggregated

[0029] The specific surface area of the silica-based particle issuitably larger than 50 m²/g, and preferably larger than 100 m²/g. Thespecific surface area can be up to 1700 m²/g, and preferably up to 1000m²/g The specific surface area of the silica-based particle is usuallywithin the range from 300 to 1000 m²/g, more preferably from 500 to 950m²/g.

[0030] The specific surface area can be measured by means of titrationwith NaOH as described by Sears in Analytical Chemistry 28(1956), 12,1981-1983 and in U.S. Pat. No. 5,176,891. The given area thus representsthe average specific surface area of the particles

[0031] The ratio between the silica-based particles and the anionicorganic polymeric particles solids in the aqueous composition can befrom about 20:1 to about 1:50, usually from 15:1 to 1.40 preferably fromabout 10:1 to about 1:30, more preferably from about 5:1 to 1:20.

[0032] The amount of anionic particles (anionic organic polymericparticles and anionic silica-based particles) contained in the aqueouscomposition is at least 0 01% by weight calculated on the total amountof the aqueous composition, preferably 0 05% by weight, more preferably0.1% by weight. The amount of anionic particles is suitably less than45% by weight calculated on the total amount of the aqueous composition,preferably 35% by weight, and most preferably 30% by weight. Suitablythe content of the anionic particles is within the range of 1 to 45% byweight, preferably within the range of 2 to 35% by weight, mostpreferably 5 to 30% by weight.

[0033] The aqueous composition has a charge of at least 0.5 meq/g,suitably the charge is within the range of 1 to 18 meq/g, preferablywithin the range of 2 to 15 meq/g, and more preferably within the rangeof 3 to 12 meq/g.

[0034] The aqueous composition of anionic silica-based particles withanionic organic polymeric particles is further successfully used as aflocculating agent in water treatment or in the treatment of wastewaterand waste sludges, which render environmental benefits

[0035] According to the present invention there is also provided amethod for preparing the aqueous composition as defined above. Themethod comprises mixing colloidal anionic silica-based particles withanionic organic polymeric particles obtainable by polymerisingethylenically unsaturated monomers together with a polyfunctionalbranching agent and/or a polyfunctional crosslinking agent. Preferablythe method comprises following steps:

[0036] (i) breaking the emulsion of the anionic organic polymericparticles to an inverse emulsion,

[0037] (ii) diluting the inverse emulsion of the anionic organicpolymeric particles,

[0038] (iii) optionally diluting the solution of the anionicsilica-based particles, and

[0039] (iv) mixing the anionic organic polymeric particles with theanionic silica-based particles, and

[0040] (v) optionally adding stabilisers to the aqueous composition.

[0041] Suitable anionic organic polymeric particles and colloidalanionic silica-based particles used in the method include those definedabove. Preferably the silica-based particles are contained in a solhaving an S-value usually in the range of from 5 to 60%, suitable from 8to 50%, preferably from 12 to 45% and more preferably from 15 to 40%before mixing with the anionic organic polymeric particles as definedabove. The S-value can be measured and calculated as described by Iler &Dalton in J. Phys. Chem. 60 (1956), 955-957. The S-value indicates thedegree of aggregate or microgel formation and a lower S-value isindicative of a higher degree of aggregation. The sols containingsilica-based particles can be modified with aluminium and/or boron asmentioned above.

[0042] The ratio between the silica-based particles and the anionicorganic polymeric particles is defined above. Also the amount of anionicparticles (anionic organic polymeric particles and anionic silica-basedparticles) contained in the aqueous composition is defined above.

[0043] The present invention relates to a process for the production ofpaper from an aqueous suspension containing cellulosic fibres, andoptional fillers, which comprises adding to the suspension a cationicorganic polymer and the above described aqueous composition containinganionic organic polymeric particles and colloidal anionic silica-basedparticles

[0044] The cationic organic polymer according to the invention can belinear, branched or cross-linked. Preferably the cationic polymer iswater-soluble or water-dispersible. Examples of suitable cationicpolymers include synthetic organic polymers and cationicpolysaccharides.

[0045] Examples of suitable cationic synthetic organic polymers includeacrylate- and acrylamide-based polymers, as well as cationicpoly(diallyl dimethyl ammonium chloride), cationic polyethylene imines,cationic polyamines, polyamidoamines and vinylamide-based polymers,melamine-formaldehyde and urea-formaldehyde resins.

[0046] Examples of suitable cationic polysaccharides include starches,guar gums, oelluloses, chitins, chitosans, glycans, galactans, glucans,xanthan gums, pectins, mannans, dextrins, preferably starches and guargums. Examples of suitable cationic starches include potato, corn,wheat, tapioca, rice, waxy maize, barley, etc.

[0047] Cationic starches and cationic acrylamide-based polymers arepreferred polymer components, and they can be used singly, together witheach other or together with other polymers

[0048] Suitable dosages counted as dry substance based on dry pulp andoptional filler, of the cationic polymers in the system are 0.1-50 kg/t(kg/tonne. “metric ton”) polysaccharide, preferably 0.1-30 kg/t and morepreferably 1-10 kg/t; 0.01-15 kg/t synthetic organic polymer, preferably0.01-10 kg/t and more preferably 0.1-2 kg/t.

[0049] Suitable dosages counted as dry substances based on dry pulp andoptional filler, of the anionic aqueous composition defined above in thesystem are 0 01-15 kg/t, preferably 0.01-10 kg/t anionic organicparticles, and more preferably 0 05-5 kg/t.

[0050] Suitable mineral fillers of conventional types may be added tothe aqueous cellulosic suspension according to the invention. Examplesof suitable fillers include kaolin, china clay, titanium dioxide,gypsum, talc and natural and synthetic calcium carbonates such as chalk,ground marble and precipitated calcium carbonate (PCC).

[0051] Further additives that are conventional in papermaking can ofcourse be used in combination with the chemicals according to theinvention, for example anionic trash catchers (ATC), wet strengthagents, dry strength agents, optical brightening agents, dyes, aluminiumcompounds, etc. Examples of suitable aluminium compounds include alum,aluminates, aluminium chloride, aluminium nitrate and polyaluminiumcompounds, such as polyaluminium chlondes, polyaluminium sulphates,polyaluminium compounds containing chloride and/or sulphate ions,polyaluminium silicate sulphates, and mixtures thereof. Thepolyaluminium compounds may also contain other anions than chlorideions, for example anions from sulfuric acid, phosphoric acid, organicacids such as citric acid and oxalic acid. When employing an aluminiumcompound in the present process, it is usually preferably to add it tothe stock prior to the polymer component and micro- or nanoparticulatematerial. Suitable amounts of aluminium containing compounds is at least0.001 kg/t, preferably 0.01-5 kg/t and more preferably 0.05-1 kg/t,calculated as Al₂O₃ based on dry pulp and optional filler.

[0052] Examples of suitable anionic trash catchers include polyamines,polymers or copolymers of quaternary amines, or aluminum containingcompounds.

[0053] Examples of suitable wet strength resins includepolyamideamine-epichlorohydrin resin (PAAE), urea-formaldehyde resin(UF) and melamine-formaldehyde resin (MF) and glyoxal-polyacrylamide.

[0054] The process of this invention is used for the production ofpaper. The term “paper”, as used herein, include not only paper and theproduction thereof, but also other web-like products, such as forexample board and paperboard, and the production thereof. The inventionis particularly useful in the manufacture of paper having grammagesbelow 150 g/m², preferably below 100 g/m², for example fine paper,newspaper, light weight coated paper, super calendered paper and tissue.

[0055] The process can be used in the production of paper from all typesof stocks, both wood containing and woodfree. The different types ofsuspensions of cellulose-containing fibres and the suspensions shouldsuitably contain at least 25% by weight and preferably at least 50% ofweight of such fibres, based on dry substance. The suspensions comprisefibres from chemical pulp such as sulphate, sulphite and organosolvpulps wood-containing or mechanical pulp such as thermomechanical pulp,chemo-thermomechanical pulp, refiner pulp and groundwood pulp, from bothhardwood and softwood, and can also be based on recycled fibres,optionally from de-inked pulps, and mixtures thereof.

[0056] The chemicals according to the present invention can be added tothe aqueous cellulosic suspension, or stock, in conventional manner andin any order It is usually preferably to add the cationic polymer to thestock before adding the anionic particles, even if the opposite order ofaddition may be used. It is further preferred to add the cationicpolymer before a shear stage, which can be selected from pumping,mixing, cleaning, etc., and to add the anionic particles after thatshear stage.

[0057] The invention is further illustrated in the following examples,which are not intended to limit the scope thereof. Parts and % relate toparts by weight and % by weight, respectively, and all solutions areaqueous, unless otherwise stated, the units are metric.

EXAMPLES

[0058] In these examples drainage and retention tests were done usingwood containing and woodfree stocks. Aqueous compositions according tothe invention were prepared from anionic organic polymeric particles(AOPP) and anionic silica-based particles, such as silica sols. Thepercentages are calculated as dry products throughout the examplesunless specified.

[0059] The compounds used in the aqueous composition of the invention,or as references are:

[0060] AOPP. anionically charged polymer particles was Polyflex CP3®available from Ciba Special Chemicals with a molecular weight around100,000 and a charge of 8.5 meq/g.

[0061] APAM anionic polyacrylamide with a molecular weight greater than10 million and about 20% charge

[0062] Silica sol: ratio SiO₂:Na₂O=45, specific surface area=850 m²/gand S-value 20%.

[0063] The composition according to the invention was prepared by firstdiluting the AOPP component and stirring for 1 hour. After that dilutedsilica sol, which is stirred for 5 to 10 min at 500 rpm before it isadded to the AOPP. The reference mixture of APAM and silica sol wasprepared by the same procedure.

Example 1

[0064] In this example, measurements of fines retention were made usinga Britt Dynamic Drainage Jar (DDJ) fitted with a vaned jar. A 200 meshscreen was employed for the retention testing and a rotor speed of 1000rpm. The fines content is comprised of a combination of filler and woodderived fines.

[0065] Initially a total fines content determination was carried out bytaking 100 ml of test stock and mixed with 400 ml of water and added tothe jar with the rotor set at 1500 rpm. By removing the drain plug thefines were washed out and discarded and a further 500 ml of water addedto the jar By repeating this process until the filtrate was clear thefines were completely removed. The long fiber fraction was then removedfrom the jar and collected on a filter paper and dried to determine thelong fiber fraction. By measuring the stock consistency, the finesfraction consistency can be deduced and used as the basis to calculatethe fines retention. The fraction of fines can also be expressed as apercentage of the total test stock consistency.

[0066] For the retention test itself 500 ml of well mixed stock wasadded to a cleaned DDJ and agitated at 1000 rpm. By using a timingsequence the chemicals were added as follows:

[0067] The stock was a wood containing stock made from mixing machinechest stock with whitewater taken from machine flume. The stock wasbased on 30% of mechanical pulp, 40% softwood Kraft and 30% broke. ThepH was 7.6. The test stock fines were 69.9% and the test stockconsistency 3.45 g/l. Cationic starch 2 5 kg/t, high molecular weightcationic polyacrylamide (CPAM) 0.37 kg/t and water-soluble AOPP 0.3kg/t. With no chemicals present the clock was started and chemicalsadded via syringe to the agitated stock in the jar; cationicstarch—after 15 seconds; CPAM—after 25 seconds, AOPP—after 30 seconds,and silica sol—after 35 seconds.

[0068] After 45 seconds the drain plug was removed and the first 100 mlof filtrate collected. The consistency of this filtrate was determinedand a retention of fines was calculated using the known totalconsistency of fines in the original test stock.

[0069] To evaluate the performance of the composition according to theinvention a composition comprising AOPP and silica sol was added after30 seconds of stock mixing in the timing sequence with the cationicpolymers added as above.

[0070] The performances of the aqueous composition of the invention andof the separate additions of AOPP and silica sol on retention of finesare summarised in Table 1. TABLE 1 Silica sol Silica sol + AOPP Silicasol + AOPP dosage separate additions composition kg/t Retention of fines% Retention of fines % 0 68.8 68.8 0.5 66.3 74.5 0.75 69.6 82.9 1.0 70.485.4

Example 2

[0071] The above Example was repeated with the exception that a woodfreestock was used. The stock was baseloaded with 10 kg/t of cornstarch andthe pH was 7.8. The fines content was 47.4% and the stock consistencywas 6.7 g/l. The stock was sampled from thin stock feeding the cleaners.The stock was based on 30% of softwood Kraft, 70% of hardwood Kraft, 10%of broke with 10% filler (PCC) Addition sequence: CPAM—AOPP—Anionicsilica.

[0072] Both AOPP and Anionic Polyacrylamide (APAM) dosages were on a drybasis The clock was started and chemicals added via syringe to theagitated stock in a cleaned jar; cationic polyacrylamide after 25seconds; anionic polymer after 30 seconds (AOPP or APAM); and anionicsilica after 35 seconds.

[0073] The retention of fines was determined as previously for the woodcontaining stock after draining the first 100 ml of filtrate at 45seconds. A composition of silica sol and AOPP or a mixture of silica soland APAM were evaluated in these tests, the composition or the mixturewas added after 30 seconds of stock mixing in the timing sequence.

[0074] The performance of the compositions comprising silica sol andAOPP or the mixture of silica sol and APAM compared to separateadditions of the components together with 10 kg/t of starch and 0.25kg/t CPAM (dry) are summarised in Table 2. TABLE 2 Fines SampleRetention % 0.3 kg/t AOPP; alone 45 0.3 kg/t AOPP + 0.1 kg/t silica sol;separate additions 53 0.3 kg/t AOPP + 0.1 kg/t silica sol; compositionof the 68 invention 0.3 kg/t APAM; alone 46 0.3 kg/t APAM + 0.1 kg/tsilica sol; separate additions 53 0.3 kg/t APAM + 0.1 kg/t silica sol;mixture 56

Example 3

[0075] In this example the drainage measurements were made usingCanadian Standard Freeness test. One liter of the diluted test stock wasused with a target consistency of 3 g/l and agitated in the DDJ at 1000rpm under identical conditions and timing sequences as with the finesretention determination above. The dilution was with tap water at roomtemperature At 45 seconds the stock was transferred to the freenesstester and the drainage test was conducted. The above mentioned stockwas the wood containing stock.

[0076] The performance of the composition comprising AOPP and silica solcompared to separate additions of silica sol and AOPP on drainage aresummarised in Table 3. The dosage of AOPP was the same in all tests andwas 0.3 kg/t. TABLE 3 Silica sol Silica sol + AOPP Silica sol + AOPPdosage separate additions composition kg/t Drainage CSF ml Drainage CSFml 0 68 68 0.25 84 87 0.5 92 103 0.75 99 121 1.0 114 134

Example 4

[0077] Example 3 was repeated with the exception that the woodfree stockwas used. The performance of the composition containing silica sol andAOPP on drainage time was compared to the performance of the mixturecontaining silica sol and APAM. The drainage times are summarised inTable 4. “Anionic Polymer” refers to either APAM or AOPP. The silica soldosage was 0.2 kg/t. TABLE 4 Anionic polymer Silica sol + APAM Silicasol + AOPP dosage reference mixture composition kg/t (dry) Drainage CSFml Drainage CSF ml 0.25 166 250 0.3 160 216

1. An aqueous composition comprising anionic organic polymeric particlesand colloidal anionic silica-based particles, the anionic organicpolymeric particles being obtained by polymerising one or moreethylenically unsaturated monomers together with one or morepolyfunctional branching agents and/or polyfunctional crosslinkingagents, the anionic organic polymeric particles and the colloidalanionic silica-based particles being present in an amount of at least 001% by weight based on the total weight of the aqueous composition. 2.The aqueous composition according to claim 1, wherein the weight ratioof the colloidal anionic silica-based particles to the anionic organicpolymeric particles ranges from 20:1 to 1:50.
 3. The aqueous compositionaccording to claim 2, wherein the weight ratio of the colloidal anionicsilica-based particles to the anionic organic polymeric particles rangesfrom 5:1 to 1:20.
 4. The aqueous composition according to claim 1,wherein the colloidal anionic silica-based particles are aluminiummodified silica.
 5. The aqueous composition according to claim 1,wherein the anionic organic polymeric particles comprise one or moremonomers selected from the group consisting of (meth)acrylic acids,alkyl (meth)acrylic acids, sulfoalkyl (meth)acrylic acids, unsaturateddicarboxylic acids, sulfonated styrenes, sulfoalkyl (meth)acrylamides,acrylamides, N-alkyl acrylamides, N,N-dialkyl-acrylamides,methacrylamides, vinylacetate, acrylonitrile, N-vinyl methylacetamideand N-vinyl pyrrolidone, and salts of said acids.
 6. The aqueouscomposition according to claim 1, wherein the polyfunctional branchingagent and/or the polyfunctional crosslinking agent has at least twoethylenically unsaturated bonds or an ethylenically unsaturated bond anda reactive group.
 7. The aqueous composition according to claim 6,wherein the polyfunctional branching agent and/or the polyfunctionalcrosslinking agent has at least two ethylenically unsaturated bonds. 8.The aqueous composition according to claim 1, wherein the colloidalanionic silica-based particles have a specific surface area within therange of from 300 to 1000 m²/g.
 9. The aqueous composition according toclaim 1, wherein the colloidal anionic silica-based particles have aparticle size in the range of from 1 to 10 nm.
 10. An aqueouscomposition comprising anionic organic polymeric particles and colloidalanionic silica-based particles having a specific surface area within therange of from 300 to 1000 m²/g, the anionic organic polymeric particlesbeing obtained by polymerising one or more ethylenically unsaturatedmonomers together with one or more polyfunctional branching agentsand/or polyfunctional crosslinking agents, the anionic organic polymericparticles and the colloidal anionic silica-based particles being presentin an amount of within the range of from 1 to 45% by weight based on thetotal weight of the aqueous composition.
 11. The aqueous compositionaccording to claim 10, wherein the weight ratio of the colloidal anionicsilica-based particles to the anionic organic polymeric particles rangesfrom 20:1 to 1:50.
 12. The aqueous composition according to claim 10,wherein the weight ratio of the colloidal anionic silica-based particlesto the anionic organic polymeric particles ranges from 5:1 to 1:20. 13.The aqueous composition according to claim 10, wherein the colloidalanionic silica-based particles are aluminium modified silica.
 14. Theaqueous composition according to claim 10, wherein the anionic organicpolymeric particles comprise one or more monomers selected from thegroup consisting of (meth)acrylic acids, alkyl (meth)acrylic acids,sulfoalkyl (meth)acrylic acids, unsaturated dicarboxylic acids,sulfonated styrenes, sulfoalkyl (meth)acrylamides, acrylamides, N-alkylacrylamides, N,N-dialkyl-acrylamides, methacrylamides, vinylacetate,acrylonitrile, N-vinyl methylacetamide and N-vinyl pyrrolidone, andsalts of said acids.
 15. The aqueous composition according to claim 10,wherein the polyfunctional branching agent and/or the polyfunctionalcrosslinking agent has at least two ethylenically unsaturated bonds. 16.The aqueous composition according to claim 10, wherein the colloidalanionic silica-based particles have a specific surface area within therange of from 500 to 950 m²/g.
 17. The aqueous composition according toclaim 10, wherein the colloidal anionic silica-based particles have aparticle size in the range of from 1 to 10 nm
 18. A method forpreparation of an aqueous composition, which comprises mixing colloidalanionic silica-based particles with anionic organic polymeric particlesobtained by polymerising one or more ethylenically unsaturated monomerstogether with one or more polyfunctional branching agents and/or apolyfunctional crosslinking agents, thereby obtaining an aqueouscomposition containing the colloidal anionic silica-based particles andthe anionic organic polymeric particles in an amount of at least 0.01%by weight based on the total weight of the aqueous composition.